The enantioselective total syntheses of two biologicallysignificant natural products, azaspiracid and amphidinolide B,, are detailed in this proposal. The azaspiracids have been shown to possess considerable toxicity in mice. During the course of this project, the absolute stereochemical assignments of azaspiracid will be proven by total synthesis. In addition, the stereochemical correlation between the C,-C25 and the C26-C47 regions of azaspiracid will be established. The amphidinolides have been shown to be potent cytotoxic agents against several cancer cell lines. Several novel methodologies will be introduced during the course of the total syntheses. These methods will include a domino [3+2] cycloaddition, tandem SO2 extrusion / Cope rearrangement, and asymmetric selenide oxidation with in situ [2,3] sigmatropic rearrangement in order to introduce key functionalities.